Certain steam generators or boilers used in power plants are subject to frequent shutdown and startup. For example, concentrated solar thermal power plants using power tower technology depend on solar energy to operate during the daytime while shutting down in the night (referred as shutdown period).
Such concentrated solar power plants use solar boilers placed at a top of a tower to produce steam used to operate steam turbines for the production of electricity utilizing generators. Generally, a solar boiler may, apart from various other components, include an evaporator section and high temperature components, such as a superheater section and/or a reheater section. The evaporator section produces steam and supplies it to the high temperature components, such as the superheater section, which superheats the steam to supply relatively high temperature superheated steam for operating the steam turbine. Each of the evaporator section or the superheater section includes various fluidically connected panels, which are heated by focusing sunrays from a heliostat field thereon, thereby heating fluid to be utilized to produce electricity.
During normal operation, the high temperature components, such as the panels of the superheater section, reach their operating temperature, and during the shutdown period they lose heat due to ambient convective cooling and radiative cooling to a relatively lower or higher residual temperature than that required for starting up the power plant in the morning. As such, starting up of the power plant without any preparation may lead into various problems such as fatigue damage of the superheater panels due to substantial differences between the temperature of the steam coming from the evaporator section and the metal temperature of the superheater components. This temperature difference between the steam and superheater components may generate through wall temperature gradients therein causing thermal stress.
In order to balance such thermal stress and improve the life/reduce the fatigue of superheater panels or components, conventionally, auxiliary steam is circulated therethrough in order to preheat or precool the superheater components. The amount of auxiliary steam required is very small as compared to the total steam generation capacity of the solar receiver. A conventional source of auxiliary steam in power plants is from a fossil fuel fired boiler or from an electric boiler. However, problems associated with such fossil fuel boilers are the use of fossil fuels and the variable operating costs based on fossil fuel usage. Fossil fuel use increases carbon emissions of the power plant. Electric boiler use increases the use of parasitic power. Solar thermal power plants have a maximum cap limit for fossil fuel or parasitic power usage. Further, substantial cost and thermal inertia are associated with piping installed running from a bottom to a top of the tower. Moreover, in event of a cloudy season, the solar receiver operates at a relatively high pressure, usually making introduction of auxiliary steam difficult. This is because in such a case the solar boilers operate at a higher pressure than those of the fossil fuel or electric boilers. Depressurization of the solar boiler may lead to significant delays in the subsequent start-up thereof. Further, if the evaporator section is isolated from the superheater section and the pressure therebetween is different, it may be difficult to establish connection therebetween.
Accordingly, there exists a need to preclude conventional techniques of auxiliary steam generation to preheat and precool the superheater section, which does not rely on a fossil fuel fired or electric boiler for auxiliary steam supply.